This invention relates generally to the field of telephony, and more particularly to an improved form of secondary or backup protection for individual subscriber pair protector modules normally installed upon a main frame in a telephone office.
Such protector modules traditionally employ a pair of carbon electrodes which arc upon the occurrence of momentary excess voltage overloads to ground the individual circuit. Later developments include heat-sensitive devices employing a fusible solder member which, upon fusing, allows a resilient member to permanently short the module to ground.
A still later development, now in widespread use, is the so-called three-element gas tube in which momentary voltage surges cause the tube to become conductive to short the surge to ground, and in which, upon the occurrence of a sustained current overload, the tube develops sufficient heat to activate a separate heat-sensitive device to cause permanent shorting to ground.
With the development of requirements for protector modules of ever smaller dimensions consistent with connector blocks having ever higher circuit densities, the use of conventional heat sensitive devices becomes more difficult because of space limitations, and it has become necessary to provide a heat-sensitive element of sufficiently simple construction to supplement the action of the three-element gas tube without requiring significant additional volume within the protector module housing. Further, in some cases, there is a requirement for secondary air gap protection which will provide protection against momentary overloads in the event of a defective gas tube in which the conductivity voltage levels are other than standard, apart from the function of the heat-sensitive element.
In the above-identified copending application, there is disclosed a construction which accommodates these requirements. This construction is of substantial utility, and has found significant acceptance in the industry. Its principal shortcoming lies in the fact that gas tube protection is normally operative in voltage ranges which are far above those occurring in the case of so-called sneak currents which are commonly present in buildings in which the modules are installed. Such currents, are often of unknown origin, and normally range in voltage between a few volts and 200 volts, although they can occur at voltages of greater value. Most of such currents are transitory in nature, but they are capable of damaging solid-state equipment connected to the subscriber side of the line. It is known to employ diodes in parallel with other protective elements for dealing with such sneak currents, and the use of specialized heat coils for this purpose is not unknown. Where a large number of individual protector elements are employed in a single module with parallel interconnections, there has arisen the problem of connection of such protective elements to a common ground in a manner which will not complicate manufacture and assembly of the module. This aspect of construction becomes increasingly important when it is realized that much of the assembly work is often performed by relatively unskilled labor.